In general, any electronic device comprises one chip or more (e.g., of semiconductor material) on which one or more actual electronic components are integrated. The chip is typically encapsulated in a package comprising an insulating body, for example, of polymeric material, in order to be insulated and protected from the external environment. The package includes electrically conductive leads exposed from the insulating body, which leads are coupled with corresponding electrically conductive terminals, for example, contact pads of the chip. The leads act as electrical interface elements between the chip and the external environment (i.e., for the connection of the electronic components with other electric/electronic elements).
Typically, the contact pads are electrically coupled with the corresponding leads through circular cross-section or flat rectangular cross-section bonding wires of electrically conductive material. In jargon, this operation is called wire-bonding.
In general, the pads are made of an electrically conductive material comprising one or more metal elements. For example, the contact pads are usually formed by means of a metallization layer comprising aluminum (Al) or an alloy comprising aluminum (such as AlSi or AlSiCu) deposited during the manufacturing of the electronic components on the chip.
Similarly, the bonding wires and the leads are formed of an electrically conductive material comprising one or more metal elements as well. In particular, the material of the bonding wires and of the leads may be different from the material of the contact pads. For example, the use of materials comprising copper (Cu) is preferred for the manufacturing of the bonding wires and the leads due to the high conductivity (both electric and thermal) of such a metal element.
Consequently, during the manufacturing of the electronic device the operation of wire bonding comprises soldering, to the contact pads of a material (e.g., comprising aluminum), the bonding wires of another material (e.g., comprising copper).
However, the different metal elements included in such materials determine different thermal expansion coefficients (for example, aluminum has a thermal expansion coefficient that is approximately double than a thermal expansion coefficient of copper). This difference between the thermal expansion coefficients may cause a weakening of the soldering between the two materials, particularly when the electronic device is used in a system subject to high operating temperature and/or substantial temperature changes; for example, this may occur in outdoor applications and/or by Joule effect due to (possibly varying) electric currents that cross an electric connection formed by the lead, the contact pad and the bonding wire.
The weakening of the soldering is exacerbated when the electronic device is used in a system subject to mechanical stresses of not negligible intensity (for example, in automotive applications, in industrial machinery, but also in portable applications). In fact, the mechanical stresses in combination with the high operating temperatures and/or the substantial temperature changes may lead to a rupture of the soldering and to a detachment of the bonding wire from the pad (event also known as “bond lift-off”) or to the development of cracks in the bonding wire at the interface between a portion of the bonding wire soldered to the contact pad and a portion of the bonding wire separated therefrom (event also known as “heel crack”).
This weakening of the structure may cause a reduction, or even an interruption, of a flow of electric current through these bonding wires (for example, generated by input/output signals or by a power supply of the device), with the consequence of reducing an efficiency of the electronic device, down to prevent the performing of the operations for which it is designed.